This invention relates to SOFC type fuel cells, in other words solid oxide fuel cells.
A fuel cell is a system that produces electricity from hydrogen, oxygen and an electrolyte by a catalyst reaction; the electrolyte separates the anode and the cathode while allowing transfer of the ionised species.
There are several types of fuel cell, the main difference being the nature of their electrolyte (liquid, polymer among others). Among these, the solid oxide fuel cell or SOFC has many advantages, including the fact that only two phases are present, namely solid and gas. Its operating temperature (of the order of 900° C.) and the heating time are such that this cell is suitable mainly for stationary applications.
As illustrated in FIG. 1, an anode (12) and a cathode (14) are separated in SOFCs by a solid electrolyte (16) through which O2− ions produced by the cathode and necessitated by the anode can circulate:
                    ⁢                                                                                        1                  2                                ⁢                                  O                  2                                            +                              2                ⁢                                                                  ⁢                                  e                  -                                                      →                          O                              2                -                                                                                                                    H                2                            +                              O                                  2                  -                                                      →                                                            H                  2                                ⁢                O                            +                              2                ⁢                                                                  ⁢                                  e                  -                                                                              →                    H        2            +                        1          2                ⁢                  O          2                      →                            H          2                ⁢        O            +      energy      
This assembly produces a voltage of the order of 1 volt with a high efficiency.
In order to obtain higher powers for commercial use, several “cells” can be associated in series or in parallel. This assembly requires that the geometry of each element and the overall architecture should be optimised, particularly for the distribution of gases, recuperation of electricity, etc.
One of the envisaged geometries is the tubular architecture: a tube is used as a support for the cathode (at the centre) surrounded by electrolyte plus an anode layer. However, the length of the current lines that it generates causes strong losses by the Joule effect and limits the power output.
One preferred embodiment is a plane architecture. Cells then have at least two gas inlets and are composed of a stack of plane cells consisting of the triple anode/electrolyte/cathode layer separated by a bipolar plate that collects current and distributes gases.
For a fuel cell, each anode must be supplied with hydrogen and each cathode must be supplied with oxygen, which particularly in the case of SOFCs, may be pure or mixed, for example in air or in an oxygen enriched air. On the other hand, contact between oxygen and hydrogen must be avoided: they burn, which reduces the performances of the cell, and particularly the mix can explode.
Although it may be easy to avoid mixing of the two gases for a single cell, for example by means of a sealed electrolyte that prevents gas transfers, in an assembly of SOFC cells in which individual cells are stacked, it is also important to assure leak tightness between the different cells, regardless of the temperature. Conventional seals no longer act at the high temperature developed by this type of cell; glass seals have been built specially for this purpose. However, the temperatures reached make the glass seals pasty: after cooling, they are no longer leak tight for a second temperature cycle (if any) in the cell.
Thus, it appears desirable to develop cell architectures that do not need seals, the properties of the new cell assemblies being optimum at the different temperatures that occur during operation of SOFC type fuel cells.